Narly Sprite is meant to help you create and edit sprite sheets. Pretty simple.

Narly Sprite requires Gimp 2.8+ because it uses group layers to organize the frames in the sprite.

Features include:

adding/removing frames

changing the active (viewable) frame

exporting the frames as a sprite sheet (horizontal strip/grid, will be adding powers of two support soon)

flattening the sprite to a new image with each frame being its own (flattened) layer

trimming the sprite to be the smallest possible width/height

copying a layer to every frame

displaying the previous frame with transparency for reference

settings dialog that gets saved with the .xcf file

option to copy the pixels in the previous frame or to just create blank layers when making a new frame

toggle a layer's visibility in all frames

This plugin becomes the most useful when you map the Sprite->Frames entries (Delete Frame, New Frame, Next Frame, Prev Frame) to keyboard shortcuts. I currently have them mapped to ALT= and ALT- for new/delete and ALT, and ALT. for prev and next.

This plugin doesn't actual export anything as animated gifs - you'll have to do Sprite->Export->Flatten and then export that using the built-in gif exporter.

Below are some screenshots of some of the menus, as well as how the frames are organized into layers:

Create Menu:

Creation Dialog:

Sprite Menu:

Frame Layers:

The images below were all made from the same .xcf file:

Exported as a horizontal sprite sheet:

Exported as a grid sprite sheet:

Flattened and exported as an animated gif:

Narly Sprite also has a settings dialog that lets you choose whether or not the previous frame is displayed with the current frame and with how much transparency: